Use of wireless sensors is rapidly increasing in residential, commercial, and industrial applications. For example, wireless sensors are used in alarm systems for residential and commercial buildings, for wireless temperature monitoring, and to detect occupancy in rooms. Buildings incorporating temperature and occupancy detection sensors, also known as “smart” buildings, can automatically control climate systems and lighting within the building.
Using wireless sensors for building applications enables simple retrofitting of buildings without the need for wiring electrical power to the sensors. However, wireless sensors are typically powered by a battery, which can power the sensor only for a limited duration. Replacing or recharging the batteries in wireless sensors can be cumbersome. One solution is to charge the wireless sensors remotely using electromagnetic energy, for example electromagnetic waves in the radio frequency band, also referred to as radio frequency (“RF”) charging. In an RF charging system, a power transmitter broadcasts energy, which is received by an antenna and converted into electric power to charge the batteries in the attached sensor.
Although RF power transmission is designed to satisfy safety regulations, some people perceive the RF electromagnetic radiation as hazardous to human health. What is needed, therefore, are improvements in RF power transmission that reduce the potential for exposure to RF electromagnetic radiation.